GENETICS
Genes are carried on chromosomes much like a ticker tape. Each chromosome is composed of one very long DNA molecule. Regions within the DNA molecule are genes. These genes code for proteins which carry out all the processes necessary for the formation, maintenance and reproduction of the individual. Homologous chromosomes carry genes for the same traits but the genes for the trait may not be identical. Alternative forms of a gene are called alleles and they arise through mutations. Most traits have multiple alleles. Sometimes the terms allele and gene are used interchangeably. Although mutations are often undesirable, some are neutral or even beneficial. The normal variation one sees in a population such as the classroom is due to allelic variation based on neutral mutations.
We inherit pairs of alleles from our parents for each trait. These alleles reside at a specific site on a chromosome called its locus (loci, plural). There are many genes at many loci on each of our chromosomes. A single pair of genes or several pairs of genes may determine a trait.
Mendel, the monk who is considered the father of genetics, worked with the garden pea. This was a good object of study since he could control its breeding. Normally, the garden pea self-fertilizes but he could prevent self fertilization and cross (breed) two different plants by putting the pollen (sperm) of one on the stigma (female part leading to the ova) of another or he could let them self pollinate. The science of genetics did not exist during Mendel's time and he died not knowing that he had discovered some basic principles of inheritance. Farmers who bred livestock and plants knew something of the practical aspects of patterns of inheritance but no one until Mendel undertook a systematic study of breeding. The term "gene" had not been used. Mendel knew nothing about cells, mitosis or meiosis. He deduced basic principles of genetics by using mathematics (statistics) and his knowledge of pea plants.
Mendel's success also came from his limiting his investigations to only a few traits of the pea plant: flower color, seed shape, seed color, plant height, etc. He also selected plants that had different manifestations of each trait. When he crossed pure breeding (what we know now as homozygous) purple flowered plants with pure breeding white plants, he found that the resulting seeds gave rise to only purple plants. He reasoned that even though the F1 (first filial) generation were all (phenotypically) purple, they had different ancestry than their purple parent and, therefore, might not "breed true." And when he let the F1 generation self fertilize, they gave rise to both purple and white flowered plants (F2 generation) in a ratio of 3:1. He counted thousands of progeny to assure himself of this ratio. When he back crossed the purple plants to their homozygous recessive parent in a "test cross," he could tell if they were homozygous or heterozygous by whether they produced white progeny in a 1:1 ratio.
We now use the term phenotype when we speak of the appearance of an individual and the term genotype when we refer to the genes that individual carries for the trait in question. So the pure breeding purple and white parents were homozygous and the F1 generation was all heterozygous and their phenotype was purple.
Mendel made the same kinds of crosses for each of the traits he analyzed and found the same results. He came to the conclusion that each parent had two factors for each trait of which they each gave only one to the offspring. We now call these factors genes and we also know they reside on homologous chromosomes, one from each parent. We use the term homozygous for his pure breeding parents and we call his F1 generation, heterozygotes. The phenotype that appears in the heterozygote (F1) is called dominant since it requires only one copy of the "dominant"allele. The alternative phenotype requires two copies of the other allele to be expressed and is referred to as recessive.
Mendel went on to do "dihybrid" crosses where he looked at the inheritance of two traits at a time. Again he crossed pure breeding parents and then their offspring (F1) were allowed to self pollinate. He consistently saw a ratio of 9:3:3:1 in the resulting F2 generation and reasoned backwards to say that each trait assorted independently. This independence of the inheritance of different traits is true only if they genes for the traits are on different chromosomes or are very far apart on the same chromosome. If two genes are close together, they tend to "go together" into the gamete. This is because crossing over rarely occurs between them when they are physically close together. Genes on the same chromosome are said to be "linked." Mendel may have tried some crosses where the genes were linked and, if so, his law of independent assortment would not have held. This could be why he stopped doing experiments and became the Abby of his monastery!
Humans have 24 linkage groups. We have two chromosomes, the X and the Y which are the sex determining chromosomes and are therefore called the sex chromosomes. However, the X and Y chromosomes have genes for traits other than sex. The genes on the X form one linkage group and the genes on the Y form another linkage group. The other 22 pairs of chromosomes are called autosomes. The genes on chromosome one form one linkage group, the genes on chromosome two form another linkage group, etc. So there are 22 autosome linkage groups plus the X and Y linkage groups to give you a total of 24 linkage groups.
There are some common misconceptions about genetic that should be corrected. Genetic does not mean unchangeable. For example, hair color is genetic but one can dye ones hair. Also, children born with inborn errors of metabolism can be put on special diets to correct the genetic disorder. The genotype cannot be changed but the phenotype may be changed.
The term, congenital, refers to conditions with which one is born. Some congenital defects are genetic and some are not. Some congenital disorders can be due to teratogens (environmental agents which cause the embryo to develop incorrectly). Also there are genetic conditions such as Huntington disease (which causes dementia and involuntary movements) which has an onset later in life, usually in the forties or fifties. This condition is not congenital but it is genetic. Therefore, not all congenital disorders are genetic and not all genetic disorders are congenital.
Some people confuse dominant traits with prevalent. Dominant refers to a pattern of inheritance of a trait and not the proportion of people with the trait. Also, people with dominant traits do not pass the trait onto all of their children. They are usually heterozygous and only 50% of their children would be expected to be affected. Sometimes people who inherit the gene for a dominant trait may not express it. This is known as incomplete penetrance. Polydactyly (extra fingers and/or toes) is an autosomal dominant trait. Only 50% of the children of a person with polydactyly would be expected to be affected. However, some people inherit the gene but never express it phenotypically. They may pass the gene to a child who does express the polydactyly phenotype.
Only sex-linked or X-linked recessive traits "skip generations." That is because genes on the X chromosome are expressed in the male but usually not in the female since she has another X which will usually have the normal allele. X-linked traits are passed by the mother to 50% of her sons. 50% of her daughters will be carriers who can also pass the gene to their sons.
Traits can be genetic without having a simple all or none effect. Eye and hair color and height are examples of genetic traits which involve many pairs of genes. These traits are called multifactorial to describe the fact that several gene pairs are involved along with environmental influences in their expression.